A synthetic aperture radar (SAR) system of a recent satellite requires a sub-meter image resolution and uses a bandwidth of over 500 MHz. Thus, a wideband chirp signal generator is needed to support a high-quality image resolution in the satellite. As a bandwidth increases, imbalance components generated in a radio frequency (RF) device decrease a quality of signal. To increase the quality of signal, a system having improved performance needs to be selected.
An ideal chirp signal refers to a signal in which a frequency changes linearly with time and a waveform has few errors caused by an insufficient number of representing bits and quantization noise.
A related art uses a memory map scheme to generate a chirp signal at a satellite. An ideal waveform is stored in a memory and generated using an address counter. However, as a signal bandwidth required for a high-resolution image increases, a required memory increases and it takes a time to switch the waveform. Research is being actively conducted to use a waveform generating method using a direct digital synthesizer (DDS) requiring a small memory and providing a fast switching speed. When the DDS is used, a linearity of an amplitude or phase may not be maintained due to the quantization error and a phase truncation error as a high frequency is generated. A pure waveform of a chirp signal to be used for satellite observation is significant. When such issues occur, a cost may increase, and thus a frequency to be generated may be restricted.
To generate a chirp signal using a DDS, as shown in FIG. 1, a timing logic may be configured based on a chirp rate and a system clock, and a chirp signal may be generated within a lamp timing signal corresponding to the chirp rate. Further, a control word for a start frequency and an end frequency of the chirp signal may be set, and a frequency step may be calculated and set to increase the frequencies based on the chirp rate. To adjust the phase and amplitude of the generated chirp signal, an adder and a multiplier may be used for a phase accumulator and a phase-to-amplitude converter, respectively.
The technology of FIG. 1 to be used for an imaging radar is suitable for generating an up-chirp signal and a down-chirp signal. However, in a case of generating a bidirectional chirp signal, a start phase of a waveform may be fixed. Thus, in a case of generating a waveform the same as the ideal chirp signal, phases may not match. The bidirectional chirp signal in which an up-chirp signal and a down-chirp signal coexist requires a time parameter which starts with a negative sign. When a control word with a negative sign is input into a DDS, the generated signal may be inverted 180 degrees. Further, in general, a phase is corrected using a complement of “2”. However, since an output phase of the DDS starts from “0”, an unnecessary error may occur. To compensate for the error, a number of complex exceptions are to be processed, and input information may need to be configured by performing an operation with respect to each waveform to be generated using an external processor. In particular, a phase offset value to be used to adjust the phase may be calculated differently depending on a characteristic of a target signal, and a definite method therefor has not been suggested.